Customized template for individual posts

I created a simple custom theme for a site that I built, which took the blog section off the home page, and re-routed individual blog posts to show they were under the /blog/ subdirectory. My only issue is that I also wanted the template of individual posts to match the template of all other pages, showing zero difference in the header, although of course some elements in an “article” (individual post) were unique to that page (like, name of author, date posted, etc.) But I can’t seem to figure out how to make the look and feel of the individual post match the rest of the site. See the difference between these two URLs:

collection of blog posts
individual post

My goal was to have the header on each individual post look no different than it does on the collection or any other page of the site. So I created a custom template that matched the template used for the home and blog pages (customized home.hbs, pre-existing index.hbs), naming it blogpost.hbs. Then I inserted the same helpers and markup that would pull the content of the post into it, borrowing from post.hbs. I then customized my routes.yaml to use this custom template for individual posts.

Nothing happened. Any ideas how I can get this to work?